One More, One Less

One more guest arrives at your party, so you fetch one more chair. A friend goes home, so that's one less. Adding or taking away just one is something you do every single day.

Every number has a neighbour just before it and just after it. One more means take a single step to the right on the number line; one less means a single step to the left. So one more than 7 is 8, and one less than 7 is 6.

n + 1 \quad\text{and}\quad n - 1

This is exactly the hop you already use when you count: counting on is just saying "one more" again and again, and counting back is saying "one less" again and again. One more, one less is simply one of those steps, on its own.

See it: a number and its two neighbours

Here is a number sitting on the line, with its one less neighbour on the left and its one more neighbour on the right. Each is a single step away. Press Refresh to try a brand-new number.

Four ducks are lined up at the pond: duck duck duck duck Now one more duck waddles in: duck You do not start counting from one again — you already had 4, so you just say the next number: five. That shortcut is the whole idea of "one more".

Bigger hops: ten more, ten less

Something neat happens when you jump by ten instead of one. Because of place value, ten more and ten less leave the ones digit completely alone and only change the tens digit. Ten more than 34 is 44 — the 4 ones never moved, but the tens went from 3 to 4.

n + 10 \quad\text{and}\quad n - 10

A coin is one. Stack ten coins and you have a tower worth ten: coin coin coin coin coin coin coin coin coin coin Adding ten more is just adding one whole tower — the loose coins on the side never change. That is why ten more than 23 is 33: one more tower, the same three loose coins.

Press play below: a marker lands on a number, then hops one step right (one more), one step left (one less), and finally a big jump of ten each way — watch the tens digit flip while the ones digit stays put. Replay it for a fresh starting number each time.

Three worked examples

One more than 29. The next number after twenty-nine is 30. The ones rolled over from 9 back to 0, so a fresh ten clicks up: 29 + 1 = 30.
One less than 40. Step back once and you land on 39 — not 41. Going down across a ten, the tens digit drops by one and the ones become nine: 40 - 1 = 39.
Ten more than 23. Add a whole ten and only the tens digit moves: 2 tens become 3 tens, the 3 ones stay put, so 23 + 10 = 33.

Two traps that catch everyone at first:

One less than 10 is 9, not 11. "Less" means step left on the number line — towards smaller numbers.
For ten more than 23, only the tens digit changes: the answer is 33, not 24. The ones digit (3) does not move at all.

Khan Academy shows how adding ten changes only the tens digit here: